The machine dishwashing process comprises washing articles in a main wash cycle and rinsing them in one or more rinse cycles. A rinse aid composition is designed for use in the final rinse step of the machine dishwashing operation, separately from the detergent composition used in the main wash cycle. The rinse aid's performance is judged particularly by its ability to prevent spot and film formation on washed articles. Rinse aid compositions usually comprise an aqueous liquid containing a low-foaming nonionic surfactant, hydrotropes and an ingredient such as citric acid that can act as a builder and a pH control agent.
For many years, sodium tripolyphosphate (STP) has been used in the main wash product for machine dishwashing operation as the primary detergency builder to sequester water hardness ions (Ca.sup.2+, Mg.sup.2+). However, precipitation of STP by hardness ions can occur under underbuilt conditions which arise when an insufficient amount of STP is present in high hardness water. This situation can result in calcium phosphate deposition (scaling) on washed article surfaces. The tendency of scaling with some slow-dissolving tablet main wash products is even higher because, during the course of tablet dissolution, the wash liquor can be underbuilt if relatively high levels of hardness ions are present. The dissolution profile of the tablet is such that, in the initial stages of the wash, only part of the available phosphate will be delivered to the wash water. In addition, underdosage of other forms of product, such as liquids, powders, granulates and gels, can also cause a comparable scaling problem.
A separate problem arises from wash liquor containing STP being carried over from the main wash cycle into the rinse cycle. This carry-over results in an underbuilt or supersaturated rinse water under hard water conditions, and can lead to further scale deposition on the articles or to a reduction in the ability of the rinse water to remove previous deposition. Usually, there is a build up of scale and this deposition causes an objectionable filming, especially on glassware surfaces. Increasing temperature and water hardness increases scaling dramatically.
Regarding inhibiting scaling, U.S. Pat. No. 5,420,211 describes acid functional copolymers grafted to a polyethylene glycol backbone as detergent additives which have the property of inhibiting film formation in the main wash of machine dishwashing. However, the control of calcium phosphate scale related to underbuilt machine dishwashing conditions with an STP-built main wash product is not taught or suggested.
WO 95/32271 describes terpolymers containing carboxlic acid, 2-alkylallyl sulfonic acid and a carbohydrate derived from sugar for use in rinsing agents for dishwashing machines to prevent the formation of spots on washed articles.
DE4415804 describes terpolymers containing acrylic acid, maleic acid and vinyl alcohol and/or vinyl acetate for use in rinsing agents for dishwashing machines to prevent the formation of spots on dried crockery, glassware and cutlery.
U.S. Pat. No. 5,306,429 describes copolymers of polyamino acids as scale inhibiting agents which are said to be useful in preventing calcium phosphate scale formation when formulated in products designed for the main wash.
EP 561464 describes polyamino compounds, including polyaspartic acid and its salts, in rinse aid compositions to prevent scaling during the rinse step. However, it teaches that this rinse aid composition is particularly useful with phosphate-free main wash compositions. This qualification means that the polymer described is for inhibition of calcium carbonate scale, related to the hard water used, rather than for inhibition of calcium phosphate scale, related to underbuilt wash conditions with STP-built machine dishwashing compositions. The nature and the formation mechanism of these two types of scale are different.
EP 659873 describes an organo diphosphonic acid compound in rinse aid compositions to prevent calcium carbonate scale. Again, the control of calcium phosphate scale related to underbuilt wash conditions is not taught or suggested.
Biodegradable copolymers of itaconic acid and vinyl alcohol or vinyl acetate have been described in WO 94/17170 for incorporation in machine dishwashing and rinse aid compositions to prevent lime scale. Again, the control of calcium phosphate scale related to underbuilt wash conditions is not taught or suggested.
The prior art has not considered the calcium phosphate scale problem, especially as related to underbuilt machine dishwashing conditions arising under high water hardness. Therefore, the objectives of the present invention are the identification of scale inhibitors that are effective for inhibiting calcium/STP scale in underbuilt conditions, and particularly, the methods of their use for superior scale-inhibiting performance in machine dishwashing under underbuilt conditions.